The optical method of caustics as applied to the measurement of the J-integral in plastically deforming materials was studied numerically and experimentally. The numerical calculations modeled a cracked body loaded under Mode I, plane-stress, small-scale yielding conditions. The results were used to generate simulated optical patterns for caustics obtained from both inside and outside the crack-tip plastic zone. These caustics were compared with corresponding caustics observed experimentally. In addition, the region of dominance of the plane-stress, Hutchinson-Rice-Rosengren (HRR) field was investigated, and the effect of plasticity on the accuracy of caustics from the elastic region outside the plastic zone was determined. The extent of the region of three dimensionality near the crack tip and its effect on the method of caustics were also investigated.